1. Field of The Invention
The present invention is directed to a twister probe for use in apparatus that test printed circuit boards. More particularly, the present invention is directed to a spring contact probe having a plunger that reciprocates in a barrel and rotates as it reciprocates.
2. Related Art
Spring contact probes are commonly used in testing printed circuit boards and other types of circuits that require a number of contact points to be checked for continuity or correct electrical signals. Such contact probes are received into probe receptacles that are typically fitted into apertures in a testing board. The entire board is moved toward and away from the testing subject by an electromechanical or pneumatic means. Typically, probes are mounted throughout the surface area of a testing board in rows and columns in high density arrays. A compression spring inside each probe forces the plunger against the printed circuit board.
When printed circuit boards are made and after electrical components are soldered to them, the runners are dirty. They are covered with lacquers and highly corrosive fluxes. In the past, these contaminants have been washed from the printed circuit boards by various solvents prior to testing them for proper electrical conductivity. Many of the solvents used for cleaning printed circuit boards have been restricted or banned or are being phased out under federal government regulations designed to improve the general environment. One such family of solvents, is for example, fluorocarbons. As a consequence, circuit boards are increasing being tested without being cleaned. Many of the currently used testing probes will not reliably penetrate the debris on the circuit boards and make good mechanical and electrical contact with the circuit board runners and consequently lead to erroneous indications of open circuits.
To overcome this difficulty, some manufactures have produced a so called "pin-point" probe that will usually penetrate the layer of contaminants on a circuit board. Unfortunately, however, such pin-point probes sometimes stick into the runners and will not retract when the test is completed. In theory, a probe pressed against the circuit board with sufficient force will penetrate the layers of contaminants and permit effective testing of the circuits on the board. In practice, however, the force that can be applied to a probe is constrained by the diameter of the plunger body and probe tip, and the diameter of the compression spring that urges the plunger body toward the printed circuit board. Only by making the probe tip small can sufficient pressure be exerted against the runner to ensure that the debris are penetrated by the probe tip. This, however, leads to the probe tip's sticking into the circuit board or circuit board runners.
Other efforts to permit testing of dirty circuit boards in hostile environments contaminated with free particles and corrosive chemicals have been developed. They employ some means for ensuring that the plunger of the probe will twist or rotate as the plunger reciprocates within the barrel. In such probes, the probe tip acts like a drill bit, turning or rotating along its longitudinal axis as it contacts the printed circuit board. The rotating or twisting motion of the sharp point of the probe enables the probe to cut through most debris resulting from the manufacture of the circuit board.
One twisting probe has a barrel that is opened at both ends, a circumferential crimp near the probe tip end of the barrel and a plunger within the barrel. The plunger includes a probe tip end and a twisted end that projects through the rear of the barrel. The rear of the barrel has a square opening that the twisted plunger passes through, causing the plunger to rotate along its longitudinal axis as it reciprocates within the barrel. The use of this probe is not known to the inventor and it could not be used in any standard printed circuit board testing apparatus because the portion of the plunger extending outwardly of the barrel would interfere with mounting the probe in a standard probe receptacle or socket.
Another twister probe includes a barrel having an open front end and rear end, with the probe tip comprising a portion of the plunger, which is seated within the barrel and reciprocates therein. A compression spring urges the plunger outward from the barrel. The barrel includes a sleeve with a curved cam slot along its length. A small cylindrical pin is inserted into the plunger body perpendicular to the longitudinal axis of the plunger body. The pin acts as a cam follower that follows the curved cam surface of the slot in the sleeve, or the enlarged portion of the barrel. This style of probe suffers from significant drawbacks. First, it includes four parts, namely the barrel, the probe, the spring and the cam follower. In addition the cam itself must be cut into the side wall of the barrel. These features lead to excessive manufacturing costs. Further, and more seriously, the cam slot can be easily contaminated by particles of lacquer, fluxes, and solder that are broken free of the printed circuit board during testing. In particular, even small amounts of flux that contact the upper exposed portion of the plunger in the cam slot can be carried into the interior of the barrel when the plunger reciprocates, and will corrode the inner portions of the probe sufficiently to produce intermittent open circuits, thus rendering the probe unreliable. Exacerbating these contamination problems, such probes are generally mounted below the subject circuit board, that is, with the probe tip pointing up. Thus, the debris loosened by the twisting probe tip falls down onto the probe itself.
An effort to overcome a related problem is found in U.S. Pat. No. 3,458,851 issued to Webb as Administrator of the National Aeronautics and Space Administration on July 29, 1969 for Electrical Connector Pin With Wiping Action. Webb '851 discloses an electrical connector pin having a spring biased cylindrical connector pin to be received within a female socket. Foreign matter in the socket may prevent good electrical contact with a connector pin that is inserted straight into the socket. Webb '851 reduces the likelihood of this occurrence by causing the connector pin to rotate as it is extended from its housing by the biasing spring, causing the connector pin to wipe the side wall of the cylindrical socket as the connector pin enters the socket.
Webb '851 accomplishes this rotation of the connector pin as it reciprocates within the housing by having an indented boss in the housing that rides in a helical groove along a portion of the connector pin shaft that is retained within the housing. There is only one indented boss, and only one helical groove. The camming action of the indented boss riding in the helical groove causes the connector pin to rotate as it reciprocates. The rear of the tubular housing is closed and sealed. The front end of the tubular housing is open to allow the connector pin to extend beyond the housing and to allow air to enter and to escape from the housing as the connector pin reciprocates therein. An inwardly extending circumferential flange at the front edge of the tubular housing forms a stop that prevents a larger diameter connector pin portion from moving outside the housing.
Webb '851 might appear to provide a good solution to the problem at hand, namely, testing printed circuit boards. This is not the case. Webb 851 includes a number of characteristics that make it unsuitable for testing printed circuit boards and that would cause serious difficulties if it were adapted for such use by, e.g., replacing the rounded tip of the connector pin with a cutting tip. First, testing printed circuit boards requires many hundreds of thousands of probes. In addition, they must be manufactured to very close tolerances to perform correctly. Consequently, the cost of manufacture, the simplicity of design, and the ease of reproducing the probe accurately are all vital considerations. Webb 851 includes a helical groove in the connector pin shaft. It is difficult and expensive to cut or otherwise form grooves in shafts and it is not economically viable in this use.
Second, pointing accuracy of the probe is of the utmost importance in testing printed circuit boards. The testing apparatus is programmed to expect a specific signal from each probe in the densely packed arrays. Each probe is aimed at a precise, specific point on the printed circuit board. If one or more probes do not hit their targets when the circuit board is tested, the test results may be wrong. Circuit board runners are sometimes very narrow and are easily missed. Incorrectly aimed or pointed probes may miss the test pad altogether. Probes that do not hit their targets can incorrectly indicate the presence of either an open or closed circuit. The strike point of a probe used for testing printed circuit boards needs to be within about thirty ten-thousandths of an inch (0.0030 inch; 0.0076 cm) of the target. This degree of accuracy is not achieved by the Webb '851 connector pin, which relies on a pilot socket portion and a pilot pin tip having a smaller diameter than the connecting portion of the connector pin to locate the connector pin in the receiving socket. The use of a single indented boss riding in a single helical groove causes the connector pin to wobble as it reciprocates within the housing as the forces exerted on the connector probe by the indented boss riding in the groove are essentially perpendicular to the axis of the connector pin and, after the limit of the sideways movement of the connector pin is reached, are translated into rotational forces. Because the lateral or sideways force exerted on the connector pin is essentially constant in both magnitude and direction and the connector pin reciprocates within the housing, the connector pin can be expected to wobble throughout its entire reciprocation cycle.
Further, the pivot bearing includes two small facing bearing surfaces at the back end of the connector pin, which are largely responsible for aiming the connector pin and these bearing surfaces offer little contact area and are restricted to only the end of the connector pin. Consequently, these bearing surfaces do not provide a high degree of pointing accuracy.
Also demonstrative of the inadequate pointing accuracy of the connector pin of Webb '851 is the fact that the rear of the housing is sealed, which requires the air that must enter and escape from the interior of the housing must pass through the gap between the connector pin and the housing. This structure is wholly inadequate for the task at hand because a gap large enough to admit the necessary volume of air will also admit the various contaminants of dirty printed circuit boards.
Accordingly, there is a need for electrical spring contact probe that can reliably penetrate the dirt and debris that typically accompany unwashed printed circuit boards and that will not be subject to contamination from that debris.